Where specifically does oxygen bind in the red blood cell?

Oxygen binds specifically to ferrous iron (Fe2+) within the heme group of hemoglobin in red blood cells. Each hemoglobin molecule contains four heme units, each capable of binding one molecule of oxygen. The iron atom in the heme is in the Fe2+ oxidation state, which allows it to effectively coordinate with oxygen molecules. This binding is crucial for the transport of oxygen from the lungs to the tissues throughout the body.

Other options like He2+, Cu2+, and Mg2+ do not play a role in oxygen transport within red blood cells. Helium does not participate in biological processes involving oxygen. Copper and magnesium might have roles in various biochemical processes, but they are not involved in the direct binding of oxygen in hemoglobin. This specificity of oxygen binding to iron is fundamental to the functionality of hemoglobin and the overall process of oxygen transport in the bloodstream.